Buying too little paint means an emergency trip back to the shop (hoping the same batch is in stock). Buying too much wastes money and leaves half-tins cluttering your garage. Getting it right requires a simple calculation — one most DIYers skip, then regret. This guide walks you through exactly how much paint you need for any space.
How Paint Coverage Works
Paint is sold by coverage rate — typically expressed as square metres per litre (m²/L). This varies by paint type, finish, and the surface being painted:
| Paint Type | Typical Coverage | Notes |
|---|---|---|
| Emulsion (matt/silk) | 10–14 m²/L | Per coat on prepared surface |
| Gloss / eggshell | 12–16 m²/L | Better on smooth surfaces |
| Masonry paint | 4–8 m²/L | Porous surfaces absorb more |
| Undercoat / primer | 8–12 m²/L | Required on bare or dark surfaces |
| Ceiling paint | 10–12 m²/L | Usually one coat sufficient |
Always use the manufacturer's stated coverage as your baseline — it will be on the tin label or product specification sheet. For budget paints, real-world coverage is often 15–20% less than stated.
The Formula
Litres needed = (Paintable area m²) ÷ Coverage (m²/L) × Number of coats
Paintable wall area = (Room perimeter × Ceiling height) − Door & window areas
Step-by-Step: Calculating Paint for a Room
- Measure room perimeter: Add together all four wall lengths. For a 4 m × 3.5 m room: perimeter = (4 + 3.5) × 2 = 15 m
- Multiply by ceiling height: Standard UK ceiling = 2.4 m. Wall area = 15 × 2.4 = 36 m²
- Subtract doors and windows: Average door = 1.8 m², average window = 1.2 m². One door + two windows = 4.2 m². Paintable area = 36 − 4.2 = 31.8 m²
- Choose number of coats: Typically 2 coats for coverage. New plaster or a big colour change may need 3
- Apply coverage rate: At 12 m²/L: 31.8 ÷ 12 × 2 = 5.3 litres
- Add 10% waste: 5.3 × 1.10 = 5.83 litres → buy 6 litres (two 3 L tins or one 5 L + one 1 L)
Worked Example: Living Room
| Surface | Area | Coverage | Coats | Litres |
|---|---|---|---|---|
| Walls | 31.8 m² | 12 m²/L | 2 | 5.3 L |
| Ceiling | 14 m² (4×3.5) | 11 m²/L | 1 | 1.3 L |
| Woodwork (skirting, door frames) | 8 m (linear) | see note | 2 | 0.5 L |
| Total (inc. 10% waste) | 7.8 L |
Woodwork note: estimate 1 litre per 16–20 linear metres of skirting/door frames at gloss coverage rates.
Special Situations That Change Your Calculation
Dark to Light Colour Change
Covering a dark colour with a light one typically requires 3 coats of emulsion, or 1 coat of specialist stain-blocking primer followed by 2 coats of your chosen colour. Calculate accordingly — 3 coats instead of 2 adds 50% to your paint requirement.
New Plaster
Fresh plaster is highly porous and will absorb more paint. The first coat should be a "mist coat" — emulsion diluted 10–20% with water. This seals the plaster. Calculate an extra coat and allow for higher absorption (assume 8 m²/L for the mist coat).
Textured Surfaces
Artex ceilings, rough render, or heavily textured walls absorb significantly more paint. Reduce your coverage figure by 25–30% (use 8–9 m²/L instead of 12 m²/L) and increase coats to 3.
Which Tin Sizes to Buy
Paint comes in 1 L, 2.5 L, 5 L, and 10 L tins. Buy the largest tins possible for your quantity — price per litre decreases significantly with size. If you need 5.8 litres, buying one 5 L + one 1 L costs less than two 3 L tins. Opened tins keep for 1–2 years if resealed airtight with the lid tapped firmly shut and stored in a frost-free location.
How to Ensure Colour Consistency
If buying multiple tins, ask the supplier to batch them together from the same production run. For large jobs split across days, mix all tins together in a bucket before starting ("boxing the paint") to ensure colour consistency throughout. Even tins of the same colour reference can have subtle batch variations.